磁县—大名断裂带西段晚更新世以来活动性研究

Study of the Activity Since Late Pleistocene of West Section of Cixian-Daming Faults Zone

1830年河北磁县M7■地震发生于磁县—大名断裂带西段,该断裂地震破裂和活动性的研究受到地震研究者关注。前人研究着眼于1830年磁县大地震的地表破裂,本文的研究重点是磁县—大名断裂带西段晚更新世以来的断层活动性。应用卫片、航片解译和野外地震地质调查等方法开展研究,重要地点进行探槽开挖或野外地质剖面剥落以及采样测年,确定了断层各段落破裂事件的发生年代。该断裂西段分为3个断层段落:F2为磁县—峰峰段落,F3为东田井村—陶泉乡段落,F4为韩家沟村—甘泉村段落。F2断层段大部分隐伏,为早中更新世断层。F3断层段东端在东田井村南断错距今22 ka地层,为晚更新世活动断层;F3断层段在鼓山南山村一带为全新世断层,从张家楼村到陶泉乡为推测全新世活动断层。F4断层段为全新世断层。F4断层段全部位于基岩山区,可见多处基岩断面、地震沟槽及断层眉脊等断层破裂;一些破裂面发育地衣——丽石黄衣[Xanthoria elegans(Link.)Th.Fr.],使用地衣测年方法确定这些破裂面为1830年磁县大地震地表地震遗迹。磁县—大名断裂带终止于F4西端。综合分析断裂带各个分段的破裂事件,得到磁县—大名断裂带西段活动事件时空分布,估计磁县—大名断裂带西段的晚更新世地震复发周期在6000年左右。

A M7■earthquake occurred in Cixian County,Hebei Province,in 1830,in the west section of the Cixian-Daming fault zone.Seismic rupture and the activity of this fault have been concerned by earthquake researchers.Previous studies focused on the surface rupture of the Cixian earthquake in 1830.The focus of this paper is on the fault activity since the late Pleistocene in the whole west section of the Cixian-Daming fault zone.The satellite interpretation,aerial photo interpretation,and field seismic geological surveys were applied to conduct researches,and trench excavation or exfoliation of field geological sections and sampling dating were performed at important locations,to determine the time of occurrence of ruptures in various sections of the fault.The western section of the fault is divided into three fault sections,in which F2 is the Cixian-Fengfeng section,F3 is the Dongtianjing-Taoquan section,and F4 is the Hanjiagou-Ganquan section.The F2 fault,an early-middle Pleistocene fault,is buried mostly.The eastern end of the F3 fault cuts off the 22 ka strata in the south of Dongtianjing Village,being late Pleistocene fault.The F3 fault section in the Nanshan Village,Gushan,is a Holocene fault,and the section is a suspected Holocene active fault from Zhangjialou Village to Taoquan Township.The main rupture in the F4 fault is the Holocene fault.The fault section of F4 is all located in the mountainous area of the bedrock,where many fault fractures such as bedrock sections,seismic trenches,and fault eyebrown ridges can be found.Lichens(Xanthoria)were developed on some of the ruptured surfaces.Lichen dating methods were used to determine these ruptured surfaces as the surface earthquake remnants of the Cixian earthquake in 1830.The Cixian-Daming fault zone ends at the western end of F4.A comprehensive analysis of the rupture events in each section of the fault zone was made to get the spatial and temporal distribution map of active events in the west section of the Cixian-Daming fault zone.It is estimated that the late Pleistocene earthquake recurrence period in the west section of the Cixian-Daming fault zone is about 6000 years.